Now I know you can't perfectly ban a single person from something but what I'm going to ask about is near-perfect banning systems where the player isn't changing his/her computer.
Name Banning: Clearly the person can change his/her name when they log in/create a new character.
Key Banning: Multiple keys may be used.
IP Banning: There exist IP changers.
So if the above 3 don't work, how do you successfully ban someone's computer from accessing your game?